The present invention relates to a color picture tube which includes an in-line electron gun and is suited to be operated at a high horizontal scanning frequency or a plurality of horizontal scanning frequencies.
A high resolution color picture tube is larger in number of horizontal scanning lines than an ordinary picture tube for television broadcasting, and hence is five to seven times higher in horizontal scanning frequency than the ordinary picture tube. For example, a display tube used as a video data display terminal is operated at a horizontal scanning frequency of 31.5 or 64 KHz. In a color picture tube using such a relatively high horizontal scanning frequency, a special contrivance is required for the convergence of electron beams.
In a Japanese patent application JP-A-56-76,145 filed by Mitsubishi Electric Corporation on Nov. 26, 1979, it is disclosed that in order to eliminate the deviation of the raster of an outer electron beam from the raster of a central electron beam, a magnetic field limiting element is provided, and further flanges acting as an eddy current inducing element are projected from above and below through holes for transmitting outer electron beams so as to be perpendicular to a magnetic field for the horizontal deflection of electron beams (hereinafter referred to as "horizontal deflection magnetic field").
Further, in Japanese patent application No. JP-A-60-86,736 and JP-A-60-86,737 filed by Nippon Electric Co., Ltd. on Oct. 17, 1983, it is proposed that a portion of the shielding wall of a shielding electrode is cut away, or that portion of the shielding wall of a shielding electrode which intersects a deflection magnetic field, is made small in area.
Furthermore, in a Japanese patent application JP-A-61-179,038 filed by Toshiba Corporation on Feb. 4, 1985, it is proposed that the thickness of the shielding wall of a shielding electrode is locally varied.
In a case where a color picture tube is operated at a relatively high horizontal scanning frequency, there arises the following problem. That is, that portion of the deflection magnetic field generated by a deflection yoke which exists on the cathode side, passes through the shielding wall of a shielding electrode, and thus an eddy current is induced in the conductive shielding wall by the deflection magnetic field which varies with time. Hence, the deflection magnetic field is weakened by a magnetic field generated by the eddy current. The magnetic field generated by the eddy current is perpendicular to the shielding wall, and hence affects to a central electron beam more than to outer electron beams. Thus, the distribution of horizontal deflection magnetic field is made non-uniform, and hence the convergence of electron beams cannot be realized. It is to be noted that the magnitude of eddy current is proportional to a deflection frequency.
At a horizontal scanning frequency used in a standard broadcasting system, the effect of the eddy current on the deflection magnetic field is very small, and hence the deviation of the raster of an outer electron beam from the raster of a central electron beam is so small as to produce no problem. While, a high resolution picture tube uses a large number of horizontal scanning lines, and is far higher in horizontal scanning frequency than an ordinary picture tube for television broadcasting. Accordingly, the high resolution picture tube is far greater in eddy current induced in the shielding wall of shielding electrode than the ordinary picture tube. Thus, in the high resolution picture tube, the convergence of electron beams is greatly affected by the eddy current.
According to the prior art mentioned in the above-referred Japanese patent applications, the deviation of the raster of an outer electron beam from the raster of a central electron beam can be eliminated, but there arise problems such as an increase in the number of parts used and an increase in the number of fabricating steps. Further, in a case where a portion of the shielding wall of a shielding electrode is cut away, the electrostatic shielding effect of the shielding electrode is weakened, and moreover the capability of the shielding electrode in protecting through holes for transmitting electron beams against a foreign substance peeling from the inner wall of a bulb such as carbon powder, is lowered. Furthermore, in a case where the thickness of the shielding wall of a shielding electrode is locally varied, the number of fabricating steps for forming the shielding electrode is increased.